1. Field of the Invention
The field of the invention is that of display devices projecting an image onto a semi-transparent screen. These devices allow an image to be displayed in front of the user in superposition on the external scene. One of the possible areas of use is the display of information in vehicles, more precisely in the cockpits of aircraft and, in particular, in the cockpits of aircrafts which have large window surface areas.
2. Description of the Prior Art
These display systems allow the attention of the user to be kept on the environment surrounding him/her, while at the same time giving him/her access instantaneously to additional information. In the case of the driving of a vehicle, having information displayed in the visual field of the external environment avoids the user having to search for this information on screens conventionally situated on his/her dashboard.
Various technical solutions exist allowing an image to be projected onto a semi-transparent screen. The most common solution is illustrated in FIG. 1. It consists in projecting the image produced by a projector P emitting in the visible spectrum onto the surface of a transparent scattering film E. The diffuser scatters the projected image towards the eye Y of the observer, while at the same time remaining relatively transparent and thus allowing the outside to be viewed. The compromise between transmission and scattering is not simple to find. Scattering films are dedicated to the projection of advertisements onto the display windows of stores. However, the technique is above all used at night, in other words under low ambient lighting conditions. In the daytime, this technique yields mediocre results. Indeed, as can be seen in FIG. 2, the film E scatters the sunlight S in all directions (straight banded arrows in FIG. 2). The transmission of the light is represented by a succession of chevrons in this figure. The film then appears milky, the transparency is limited, the efficiency in reflection and the brightness of the projected image remain low and basically unsatisfactory.
Accordingly, in the application FR 2 986 624 entitled “Optical projector with a semi-transparent projection screen”, the applicant provides a display screen comprising a plurality of patterns scattering the light regularly distributed over a transparent surface, where these patterns can comprise a reflecting surface coating. With respect to the previous systems, the semi-transparent screen of this projector both possesses a high transparency and yields a high brightness of the projected image, while at the same time having a limited diffraction.
However, the light rays coming from the projector and scattered by the screen all have a known angle of incidence. In the same way, the illumination by sunlight can only come from certain directions, certain parts of the cockpit forming a natural mask. However, the previous screen is isotropic and does not take into account, in a precise manner, the variations in angle of incidence of the light rays coming from the projector in such a manner as to optimize the efficiencies of scattering towards the eye of the observer.